Plasma uniformity, ion to radical flux ratio and energy flux control during plasma processing for treating semiconductor substrates are important to achieve patterning structures on a substrate or controlling the amount of material removed from or deposited on or into the substrate. Common methods for plasma generation are inductive coupling from an induction coil driven at radiofrequency power or the application of microwaves. Each method has distinct merits. Inductively coupled plasma sources (ICP) may be used to generate plasma at relatively low pressure high electron density and high electron temperature in the body of the plasma. In contrast, microwave sources tend to be operated at higher pressure. Operating in the over dense plasma regime or surface wave mode tend to generate relatively lower electron temperatures (Te) in the plasma volume and near the substrate while also providing a large ion flux to the wafer. An example of this type of surface wave microwave plasma source is the radial line slotted antenna plasma source.
With surface wave driven microwave sources, it is difficult to control the plasma uniformity at low power and low pressure, regimes where ICP sources are typically operated. Furthermore, low electron temperature is not always desirable. Hard mask open (HMO) processes are those in which a silicon nitride or silicon dioxide mask is patterned with photoresist, then etched in a CF4 or CHF3 containing plasma so that the newly patterned silicon dioxide or silicon nitride may be a mask for the polysilicon gate patterning step that follows. HMO processes benefit from large fluorine fluxes (large fluorine to fluorocarbon (CF2) ratio) from ample dissociation of precursors such as CF4 in the case of oxide mask open. Among many considerations, excessive deposition on the photoresist must be avoided for mask critical dimension (CD) control. When the electron temperature is low and the chamber residence time long, while ample amounts of fluorine are generated by the microwaves, recombination of the fluorine with or among fluorocarbon radicals results in small fluorine to fluorocarbon radical ratios incident on the substrate. While the uniformity is excellent at high pressure, the plasma chemistry conditions are not optimal for this illustrative HMO process. Low pressure plasma chemistry conditions are desirable but good uniformity is difficult to achieve. For a gate stack etch step that follows the HMO step, microwave source operation at high pressure with resultant low Te conditions are desirable for profile control and selectivity. Large radical fluxes (e.g. Cl) that occur at high pressure result in radical coverage on the substrate that leads to highly chlorinated inert by-products (e.g. SiCl4) as opposed to reactive by-products (e.g. SiCl) that redeposit on sidewalls leading to tapered polysilicon profiles. If the electron temperature is low, dissociation of the by-products into undesirable low degree of halogenation products is avoided. High pressure operation ensures that the ion energies are low enough to provide adequate selectivity when the gate is etched to the oxide beneath the polysilicon and selectivity to the hard mask.
ICP sources work well at low pressure and are characterized by high Te in the region that encompasses most of the plasma volume. As a result, conditions are desirable for HMO. The plasma uniformity is also typically acceptable. Unlike those for high pressure drive microwave discharges, gate etch processes with low pressure ICP operation require multiple process steps to avoid damage, recess in planar gate or physical damage to a fin structure, for example, in FinFET etch. The source of damage is related to the larger plasma potential near the wafer resulting from the relatively high electron temperature at low pressure.
Hence, it would desirable to combine the features of ICP and microwave sources in a way that enables processing of planar substrates.